The present invention relates to an apparatus and a method for determining the circumference of a body.
In virtually all technical areas, it is often necessary to determine or check the circumference of a body. In the area of production a circumference measurement is often part of a quality check on products. In this case, it is often important that this circumference determination is carried out as exactly as possible, but at the same time also quickly and simply, since it is most often only one measurement which accompanies the actual process and, in terms of time, is not intended to have a very detrimental effect on the fault-free progress of the process. A further application for a circumference meter is in the medical area. Here, for example in the area of venous occlusion plethysmography, it is necessary to be able to determine the exact circumference of any desired extremities of a patient.
DE 3 223 711 describes an apparatus and a method for measuring the lower extremities, for example as a basis for the preparation of medical compression hose, the circumferential measurement being carried out by using a measuring tape loop whose circumference can be varied by an electric-motor drive and which, when recording the circumferential dimension, encloses the extremities with a constant, electronically regulated contact pressure. With the aid of an electronic eye module sensing the measuring tape scale and a counter electrode, the measured result is displayed digitally. The drawback with this apparatus and this method is the complicated handling, since a closed measuring tape loop has to be slipped over or guided over the extremities to be measured. A further drawback consists in a measurement inaccuracy which increases with time as a result of the measuring tape moving to and fro during a measuring operation.
It is an object of the present invention to provide an apparatus and a method with which rapid, simple and accurate determination of the circumference of a body is possible.
According to the invention, this object is achieved by the apparatus described in the claims and by the method described in the method claims.
Within the context of the present invention, an apparatus for determining the circumference of a body is provided, the apparatus having at least the following elements:
a) a wire which has a known length section of length LD,
b) a position sensor, which is in operative contact with the wire such that it outputs a signal S, as soon as the wire exerts at least a predetermined tensile force Z on the position sensor,
c) a drive device for tensioning the length section of the length LD of the wire along the circumference of the body, the drive device being connected to the position sensor such that when the signal S is output, the tensioning of the wire by the drive device can be stopped,
d) a distance meter, in operative contact with the drive device, for measuring the distance WD traced by the wire as it is tensioned along the circumference of the body by the drive device,
e) so that the circumference of the body can be determined by the length LD, the distance WD and an apparatus-dependent geometry factor G.
It is preferable if the apparatus according to the invention has a unit for the calculation and output of the circumference of the body from the length LD, the distance WD and an apparatus-dependent geometry factor G. This ensures rapid and simultaneously exact determination of the circumference. It is preferable if the distance meter is coupled directly to a signal processing unit, preferably to a microprocessor or, particularly preferably to a computer, in which the measured distance WD traced by the wire as it is tensioned along the circumference of the body by the drive device can be read in and evaluated directly. The parameters of the apparatus which are known in advance, such as the length LD of the known length section of the wire and the apparatus-dependent geometry factor G, can be stored in the signal processing unit as fixed variables and called up at any time, or, preferably, integrated directly into a program used for the evaluation. The resulting circumference then preferably appears on a monitor or display connected directly to the processing unit.
In another preferred embodiment of the apparatus according to the invention, the drive device is a mangle mechanism with a drive roller and a traction roller. The introduction of the wire between the drive roller and the traction roller means that the apparatus becomes intrinsically self-contained. The drive device, the wire and the position sensor therefore form an intrinsically self-contained, complete system for determining the circumference of a body around which the wire wraps. After the drive roller has been set operating, the wire functions as a transmitter of the torque from the drive roller to the traction roller, so that the two rollers rotate synchronously with each other and, in so doing, simultaneously draw in the wire. The length by which the wire is drawn in between the two rollers can simultaneously be measured at any time with the aid of the distance meter coupled to the traction roller. As soon as the wire has been tensioned, that is to say as soon as it rests directly on the body whose circumference is to be measured, the wire exerts a finite tensile force on the position sensor, with which it is in operative contact. The position sensor reacts to this tensile force acting on it by generating a signal. This signal is preferably in turn coupled back to the drive roller which, when the signal appears, stops operating, so that the wire is not drawn in further by the two rollers, that is to say the drive roller and the traction roller.
The position sensor used is preferably an inductive distance sensor, whose impedance changes as a function of the force acting on it, as a result of the wire in this case. Here, the force which is necessary for the position sensor to respond, lies far below the compressibility limit of the body, whose circumference is to be determined. This ensures that the body is not compressed by the wire, and therefore changes its circumference, before the position sensor responds, that is to say provides a signal. In order to ensure that the compressibility limit of the body still has not been reached during the circumference measurement, after the first signal provided by the position sensor, the wire is preferably drawn in further by the drive device by a specific amount. The comparison of the length of the wire that is drawn in and can be read off on the distance meter after the first signal provided by the position sensor, together with the corresponding length extension of the position sensor which occurs simultaneously after the first signal as the wire is drawn in further by the drive device, offers a good possible means of checking; this is because the two distances must be identical if the body has not been compressed by the wire now resting directly on the circumference of the body.
In a preferred embodiment, the drive device has an electric motor.
In a preferred embodiment of the apparatus according to the invention, at least part of the surfaces of the drive device which come into contact with the wire as it is tensioned along the circumference of the body has a roughening which can be achieved by photomechanical or mechanical or chemical or electrochemical treatment. In the previously mentioned preferred refinement of the drive device as a mangle mechanism, such deliberate roughening of the traction roller achieves good adhesion of the wire on the traction roller and therefore ensures that the wire is drawn in simple, continuously and steadily. In the case of the mangle mechanism, good adhesion between wire and traction roller is also indispensable for its satisfactory functioning, that is to say for the interplay of traction roller, drive roller and wire.
While the adhesion between wire and traction roller should be comparatively high, the adhesive and sliding friction of the wire on the circumference of the body to be determined must be comparatively small, so that the wire can slide along on the circumference satisfactorily as it is tensioned. It is therefore preferable for a smooth wire, in particular a nylon wire, to be chosen. Furthermore, a material, in particular nylon, is preferably selected for the wire, so that the cross-sectional area of the wire remains substantially constant, even under loading in wide limits. This results in a constant contact area between the wire and the traction roller and therefore constant process conditions. The wire preferably consists of a flexible material, in order to ensure a good material fit as the wire is tensioned around the circumference to be determined. Nylon is also well-suited as a wire material with regard to this material property. A further preferred material property for the wire is a high tearing strength. This property is also satisfied by nylon. To this extent, the wire which is used to wrap at least partly around the body along its circumference consists of nylon in a preferred embodiment of the invention. Nylon is a cost-effective but, at the same time, also a very stable, in particular tear-resistant smooth and flexible material. The wire is therefore very stable with respect to tensile forces acting on it over a wide range. When the wire is laid around the body and tensioned as a result of being pulled in by means of the interplay of drive roller and traction roller, there is therefore no risk, or only an extremely small risk, that the wire will tear before the determination of the circumference could be carried out. In addition, a wire material is preferably selected whose length change as a function of the temperature is vanishingly small. This property is also satisfied by nylon.
In a further preferred embodiment of the invention, use is made of a highly sensitive sensor which reacts at a very low tensile force exerted on it by the wire, that is to say provides a signal. It is preferable if, as already mentioned, an inductive distance sensor is used here. It is therefore possible, with the aid of the present invention, even to determine the circumference of such bodies which have a finite compressibility, that is to say are not rigid. Such bodies have to be handled, for example, in the case of extremities. In the area of occlusion and/or compression phlethysmography, it is necessary to determine the circumference of extremities as a function of the state of the corresponding vessels. Depending on the state and function of the vessels, the circumference of the extremities can vary. With the aid of the present invention, these often very small but significant changes in circumference can be determined very accurately, that is to say with a resolution of about 1 xcexcm, and quickly.
In addition, the present invention relates to a corresponding method of determining the circumference of a body, the method having at least the following steps.
a) Laying a wire with a known length section of length LD around the body in the circumferential direction, the wire being in operative contact with a position sensor such that the position sensor outputs a signal S as soon as the wire exerts at least a predetermined tensile force Z on the position sensor,
b) Introducing the wire into a drive device which is in operative contact with a distance meter,
c) Tensioning the wire along the circumference of the body by starting the drive device, until the position sensor in operative contact with the wire provides a signal,
d) Reading the distance WD, determined with the distance meter, traced by the wire as it is tensioned along the circumference of the body by the drive device,
e) Determining the circumference of the body from the length LD, the distance WD and an apparatus-dependent geometry factor G.
In addition, the present invention relates to the use of the apparatus according to the invention and of the method according to the invention in the area of occlusion and/or compression phlethysmography, in particular for determining the circumference of extremities.